1. Field of the Invention
The present invention relates to a wireless communication system and, in particular, to a cell selection method and apparatus of a terminal in a Long Term Evolution (LTE) system of heterogeneous network.
2. Description of the Related Art
Recently, wireless communication technology has advanced rapidly. With this rapid advance, the communication systems have evolved. Among the recent wireless communication technologies, Long Term Evolution (LTE) is a promising 4th Generation mobile communication standard. An LTE system may include various types of base stations such as macro base station, pico base station, and femto base station according to their deployment positions or purposes. A network in which the diverse types of base stations coexist is referred to as Heterogeneous Network (HetNet). The diameter pico/femto cell is small as compared to that of the macro cell.
In the heterogeneous network, the coverage of the pico/femto base station may be overlapped with or included in the coverage of the macro cell. In this case, it is difficult to expect accurate signal measurement due to the inter-cell interference. In order to solve this problem, inter-cell interference coordination, i.e. Enhanced Inter Cell Interference Coordination (eICIC), technology is used.
Almost Blank Subframe (ABS) is one of the key techniques in the eICIC technology. In LTE, transmission is performed in unit of subframe. ABS is a technique for muting transmission in the serving cell or neighboring cell at a specific subframe. ABS aims to make it possible for the terminal to measure received signal without interference at the corresponding subframe.
FIG. 1 is a diagram illustrating the HetNet to which ABS technique of eICIC technology is applied.
Referring to FIG. 1, a femto Closed Subscriber Group (CSG) base station 103 allowing access to restricted users is located within the coverage of the macro base station 101. In the case that the non-CSG terminal 105 in communication with the macro base station 101 moves to approach the femto CSG base station 103, the non-CSG terminal 105 may undergo a problem in communication due to the interference from the femto CSG base station 103. In this case, the macro base station 101 notifies the terminal 105 of a measurement-restricted subframe pattern 107 in the form of a bitmap. If the pattern is received, the terminal 105 suspends measurement at the subframes indicated by 0 in the pattern 107 and performs measurement only at the subframes indicated by 1. The femto CSG base station 130 incurring interference does not transmit signal almost at the subframe indicated by 1 (ABS) so as to avoid interference. The terminal 105 is capable of measuring signal in the interference-mitigated environment.
The eICIC technologies in HetNet that are known so far are almost related to the terminal (UE) connected to the network. According to the conventional method, the base station can control the UE connected to the network directly so as to mitigate interference by controlling the UE. However, there is a need of applying the eICIC of HetNet to the UEs in idle mode that are not controlled by the network. In the HetNet system, the Reference Signal Received Quality/Reference Signal Received Power (RSRQ/RSRP) of the UEs in idle mode varies at every subframe, this influence the cell selection or reselection result. There is therefore a need of determining the subframe for performing cell reselection or reselection based on the signal strength measured thereat. Accordingly, there is a need of modification in UE operation.